1. Field of Technology
The present disclosure relates to a heave compensating system, and more particularly relates to a heave compensating system for a marine vessel.
2. Background Information
As is well known, the search for hydrocarbons through the seabed often involves the use of floating marine vessels such as drill-ships or floating marine platforms. The use of floating vessels of this type is generally considered advantageous over the alternative of using fixed platforms resting on the seabed during exploratory operations as they are more readily moveable from site to site.
However, vessels are subjected to upward and downward heave motions due to wave action. A coring or drilling tool is typically carried at the lower end of a string or drill pipe suspended from the vessel. During coring operations, if no compensation is made for the heaving motion of the vessel above, very substantial variations can result in the force applied to the coring tool in the seabed, and this can result in unpredictable compactions or weakenings in the core retrieved the tool, thereby destroying the core or at least reducing its effectiveness for analysis. During drilling operations, heave-induced load variations on a drill bit are known to accelerate the wear of the bit. As will be appreciated, if a vessel is caused to move in heave to an excessive degree, for example in rough sea, very significant damage can be caused to such tools. It can also be important to compensate for the heave motion of a floating vessel when performing other types of hoisting operation from the vessel.
Heave compensating systems have therefore been proposed and are generally used on such vessels to maintain a substantially constant force on the tools, and optionally to maintain the tools in a substantially constant position, as the vessel rises and falls in heave. Previously proposed heave compensator systems generally comprise a motion-compensating hydraulic cylinder associated with the crown block or the travelling block of a derrick arrangement mounted on the vessel and from which the drill string or other tool or load is suspended. The hydraulic cylinder is fluidly connected to a hydraulic accumulator that is driven by the flow of the hydraulic fluid between the cylinder and the accumulator. Such a system is purely passive in nature.
In a purely passive arrangement of the type described above, the nominal pressure charge of the accumulator determines the nominal hydraulic pressure of the compensating cylinder, which in turn determines the magnitude of the load suspended from the vessel which can be held substantially constant despite heaving motion of the vessel. The accumulator's pre-charge pressure must therefore be adjusted to balance the static load whose motion is to be compensated. However, prior art systems of this general type are known to exhibit substantial force variations due to the pressure-dependency of the accumulator on its charge. These variations may sometimes be tolerated for systems such as a so-called dead-line compensator, but may require further compensation in other systems, such so-called crown mounted compensators. In such systems, this further compensation is generally achieved via the use of mechanical, position-dependent transmissions. Nevertheless, while such arrangements can reduce the accumulator charge-dependent force variations, they cannot readily compensate for friction damping and inertia effects. It is therefore common practice to add an active heave compensator arrangement to compensate for these force variations in the passive arrangement. However, conventional combined passive/active heave compensator arrangements can be very complicated, expensive, bulky and can be limited in various operational modes.